Rooms by themselves dont have a phasing parameter that can be changed. If you move the bounding walls of a room to an existing phase, the room does not automatically go to that phase. Rooms take the phase of the view they were created on. So, to move a room to an existing phase, one has to copy the room and paste it in the view that assigned to an existing phase. Revit can have different rooms bound by the same walls (elements) as long as they are in views that are assigned to different phases.